This invention relates to a translucent or light-transmissive, alumina ceramic tube and to a method for producing the same.
Translucent alumina ceramic is frequently used as envelopes or tubes for high pressure sodium lamps. It is chemically stable, has high mechanical strength, and exhibits excellent electrical and thermal characteristics.
Because of those favorable properties, translucent alumina ceramic has various usages. However, more versatility in forms or shapes would broaden its utility.
Conventionally, translucent alumina cermic has been shaped by such methods as isostatic pressing, extrusion molding and dry pressing. Consequently, its shapes have been limited to such simple ones as tubes, plates and flanged plates. With such molding methods as heretofore employed, it has been difficult to produce a tube having a diameter differing along its length
For example, high pressure sodium lamps are difficult to seal. It is desirable, therefore, that they should have smaller diameters at the end portions to be sealed, than at the central portion. It is difficult to make the conventional tubes having such shapes by isostatic pressing, extrusion or dry pressing.
Also the envelopes or tubes for metal halide lamps are preferably made of translucent alumina ceramic, rather than conventionally employed fused quartz. A favorable shape of tube for metal halide lamp, however, is that inflated at a central portion, not a straight tube, because of the normal behaviour of an arc. Conventional methods for shapinng translucent alumina ceramic again fail to provide a product of such a configuration.
Slip casting is desirable for producing varieties in the shapes of translucent alumina ceramic. However, the following problems are encountered in attempting to apply slip casting to the production of alumina ceramic shapes:
(1) The starting material for translucent alumina ceramic is alumina powder of particles which have extremely small diameters, even the largest not exceeding 1 .mu.m. In the process of making a slip or slurry of such ultra-fine powder, it is difficult to reduce the amount of the liquid media (normally water).
Consequently the density of the cast pieces becomes low. It renders the subsequent firing or sintering of the translucent alumina ceramic difficult, so that only a porous product is produced.
(2) It order for a translucent alumina to retain sufficient transmissivity, the starting alumina powder must have a purity not lower than 99.9%. This requirement severely limits the type and amount of additives which may be used for making the casting slip or slurry.
(3) A defloccculant must be added to the slip or slurry of alumina powder. The theories on the function of the deflocculant have been based on the casting of conventional ceramics.
Casting of conventional ceramics has mostly been practiced with clay-containing systems. Thus, conventional theories on the function of the deflocculant are not applicable to a system using ultrafine powders such as alumina powder.
Again, sodium-containing compounds have been mainly used as deflocculants in the casting of ceramics free of clay, such as water glass, sodium humate, sodium alginate and PVA. Those deflocculants, however, cannot be used for the shaping of translucent alumina ceramic.
The addition of the deflocculant in the amount of 0.05-0.3% by weight, based on the powder of the deflocculant has been recommended. That amount of deflocculant, however, has been determined for powders of normal particle sizes, and does not relate to on the use of an ultrafine powder which has a drastically increased specific surface area.
Thus, the amount of deflocculant which should be added to the alumina powder was not previously clear.
(4) Prior to shaping the translucent alumina ceramic a, grain growth inhibiter is mixed into the slip or slurry. Magnesium compounds have been used for this purpose.
A magnesium compound, however, releases magnesium ions as it is dissolved in water. A substantial amount of magnesium ions act to raise viscosity of the slip or slurry, working together with the deflocculant.
When the slip or slurry viscosity increases, the solids content of the slip or slurry must be reduced. Consequently a larger amount of the deflocculant must be used to form a slip or slurry of high solids content, which is apt to result, however, in the cracking of the shaped and fired product.